Damage to the main and emergency spillways of the Oroville Dam in California, triggered in February 2017 by heavy rain that was part of Northern California’s wettest winter in almost 100 years, has brought to light several lessons that hydropower and dam engineers can learn from the disaster. The cost to repair the spillway has reached $870 million, according to the state’s Department of Water Resources (DWR), as repair work has discovered numerous defects in the original construction from the 1960s.